Cucumber-Rails is an integration testing tool for Rails 3 applications that uses Cucumber's behavior-driven development (BDD) approach. It allows customers and developers to collaborate on acceptance tests written in a natural language format called Gherkin. Cucumber tests (called "features") are high-level and focus on integration, while RSpec tests are more granular unit tests. Features are made up of scenarios written in Gherkin, while step definitions translate those scenarios into code. Cucumber can be used with Rails to improve communication, clarify requirements, and reduce bugs through automated acceptance testing.